The present invention relates to a device and a method for reading a code arranged e.g. at a package for a product, in particular for minimizing a perturbation signal induced in an electrically conductive layer in the vicinity of the code.
Identity codes comprising a set of (or a series of) code elements or symbols are commonly placed on product packages to provide information about the products contained in these packages. These identity codes can be used in e.g. supermarkets for keeping track of the products or in industry for “track-and-trace” purposes in packaging production lines. The identity code can be made of e.g. a series of parallel lines, a geometrical arrangement of dots or of concentric circles, and be comprised between a start and a stop marker. Each of the code elements usually represents one bit of information of the identity code. Using e.g. a series of lines, the width of each of the lines and the spacing between the lines can be varied to represent different bits of information of the identity code, often called bar code. The identity code can then be read and decoded to retrieve the information about the product, such as e.g. its name. In bar code systems using optical reading methods, such as laser scanning or linear imaging, the identity code is scanned or imaged, respectively, to read the information. Thus, such methods require line-ofsight, which implies that the identity code has to be placed at an exterior surface of the product package to allow scanning or imaging. However, identity codes based on visual identification are not desirable since they can easily be reproduced and thereby facilitate counterfeiting of products. Further, identity codes are preferably not located at the exterior surface of the product package because of esthetical issues and risks for damages that would result in loss of information.
An alternative technique to optical bar code systems is a technique based on capacitance measurements using capacitance coupled radiofrequency identity systems, called capacitance coupled RFID systems in the following. In this technique, the code is composed of a series of or a set of code elements where each code element typically is comprised of two electrically conductive plates, either connected to each other or not connected to each other. The reader typically has a series of, or a geometrical arrangement of, pairs of antennas corresponding to the arrangement of the code elements of the code on the package where one of the antennas of said pairs capacitively induces a communication signal into the code element, and where the other antenna receives an input signal induced in response to the communication signal, if the two plates of the code element are connected. In this case, a bit of information corresponding to a value of “1” is received. If the two plates are not connected to each other, no signal is induced in the receiver antenna and a bit of information corresponding to a value of “0” is received. As capacitive connection is used, the code can be placed inside the packaging material of a product and therefore be both hidden and protected from e.g. scratches. However, capacitance coupled RFID systems are not used in applications where electrically conductive foils are comprised in the package of the product since the reliability of the technique is considerably reduced, due to the addition of a disturbing signal to the input signals received by the reader because of the presence of the electrically conductive foils. This problem is usually solved by increasing the input signals induced in response to the communication signals, which can be achieved by employing larger antennas in the reader and larger code elements in the code.